Wet spinning of an acid solution of an aromatic polyamide followed by neutralization of residual acid yields salts, e.g., sodium sulfate, in the fibers. The presence of salt in the fibers causes undesirable deposits on apparatus used for handling the fibers. Excessive amounts of salt in the fibers are also believed to interfere with adhesion of the fibers to rubber and various resins.
In commercial spinning processes, high speed operation is essential for economic practicability. Thus the time available for coagulating and washing of wet-spun fibers is extremely limited.
Since a small amount of the above-mentioned residual salt can be tolerated, improvements in the washing process can provide either a lower salt content in the fibers or a higher spinning speed at the same salt level.
The amount of salt present is proportional to the amount of residual acid present in the fibers at the time of neutralization. Thus any improvement in the efficiency of acid extraction will provide a decrease in residual salt content after neutralization.